1. Field of the Invention
This invention relates to apparatus and a method for providing electric power to a load from alternate sources, and more particularly, to a transfer switch system which prevents connection of a source to a load bus as long as a large out-of-phase voltage such as would be produced by a highly inductive load remains on the load bus.
2. Background Information
Alternate power sources are provided for any number of applications which cannot withstand a lengthy interruption in electric power. Typically, power is provided from a primary source with back-up provided by a secondary source. Often the primary source is a utility, and the secondary source is an auxiliary power source such as an engine driven generator.
Transfer switches are used to switch between the two power sources. The transfers can be made automatically or manually. In the case of a generator driven auxiliary power source, power must be stabilized before the transfer can be made. In any event, the two power sources cannot be connected to the load simultaneously unless they are in phase. Thus, most transfer switches make an open transition transfer. That is, the previously connected source is disconnected from the load before the other source is connected. However, there can still be a problem if the load is highly inductive such as a motor which can produce a residual voltage on the load bus. The frequency of this residual voltage is a function of motor speed which decreases with time, so that the phase is continuously changing relative to the phase of the new source. While the amplitude of the residual voltage decays with time, a high initial value together with the random phase relationship can produce a vector difference resulting in high inrush currents which may damage the motor or at least trip overcurrent protective decives in the system.
The most common approach to overcoming the problem of the residual voltage produced by highly inductive loads is to provide a time delay which allows the residual voltage to fall to a safe level before the second source is connected. Typically, the new source can be connected to the load when the residual voltage has dropped below about 30% of the initial value. While this is effective, it requires providing a time delay adequate to accommodate the worst case.
There is a need therefore, for an improved apparatus and method for transferring highly inductive loads from one power source to another.
There is also a need for such an apparatus and method for making such a transfer as soon as it can be safely made.
There is also a need for such an apparatus and method for making such a transfer without having to provide for synchronization of source voltage and residual voltage produced by the load.